Reducing re evaporation of vacuum vapor deposited coatings

ABSTRACT

RELATIVELY THICK VACUUM VAPOR DEPOSITED COATING OF HIGH VAPOR PRESSURE METALS ARE PRODUCED BY PROVIDING ON THE COATING IMMEDIATELY AFTER IT CONDENSES A SURFCE LAYER OF A SUBSTANCE HAVING A VAPRO PRESSURE LOWER THAN THAT OF THE COATING METAL.

Sept. 25, 1973 v I. ss N ET AL 3,761,302

REDUCING RIB-EVAPORATION OF VACUUM VAPOR n'mmswmn COATINGS Filed July 21, 1971 Fa'gl.

their ATTORNEY 3,761,302 REDUCING RE-EVAPORATION OF VACUUM VAPOR DEPOSITED COATINGS Irwin I. Bessen, 7320 Tangle Ridge Lane, Cincinnati, Ohio 45243, and Harold E. Farrey, 126 Bancroft Road, Lansdale, Pa. 19446 Continuation-impart of application Ser. No. 12,848, Feb. 19, 1970, which is a continuation-in-part of application Ser. No. 810,910, Mar. 13, 1969, which in turn is a continuation of application Ser. No. 547,485, May 4, 1966, all now abandoned. This application July 21, 1971, Ser. No. 166,585

Int. Cl. C23c 13/00 US. Cl. 1l7-71 M 7 Claims ABSTRACT OF THE DISCLOSURE Relatively thick vacuum vapor deposited coatings of high vapor pressure metals are produced by providing on the coating immediately after it condenses a surface layer of a substance having a vapor pressure lower than that of the coating metal.

This invention relates to a process of coating a substrate with a coating metal by vacuum vapor deposition. It is more particularly concerned with a process for coating a high vapor pressure coating metal. This application is a continuation-in-part of our application Ser. No. 12,848, filed Feb. 19, 1970, now abandoned, which is a continuation-in-part of our application Ser. No. 810,910, filed Mar. 13, 1969, now abandoned, which is a continuation of our application Ser. No. 547,485, filed May 4, 1966, now

abandoned.

It is known to coat a metal substrate such as steel strip or the like with a different coating metal by vaporizing the coating metal in an evacuated chamber and causing it to condense on the substrate. Heretofore, however, it has been difficult, if not impossible, to deposit in this way coatings of high vapor pressure metals above a critical thickness, which thickness varies from metal to metal. Zinc is a particularly intractable coating metal in this respect, and although zinc coated steel is an article of commercial importance, it has heretofore not been possible to produce steel or thin sheet and tin plate gauges with a vacuum vapor deposited zinc coating of a thickness greater than about 10- inches. Such coated sheets do not have the corrosion resistance of steel sheets zinc coated by other processes.

We have discovered that the difficulties above-mentioned encountered in the vacuum vapor deposition of zinc arise from the relatively high vapor pressure of zinc at the temperature obtaining in vacuum vapor deposition processes. As the zinc vapor condenses on the substrate, it gives off heat which is absorbed by the substrate, thus raising its temperature. This effect is, of course, most pronounced with thin substrates. The vapor pressure of zinc increases with increasing temperature. As its vapor pressure increases, the zinc tends to re-evaporate from the substrate. Thus an equilibrium condition is approached in which the condensation of additional zinc merely results in the re-evaporation of a corresponding amount of zinc.

The thickness of the coating metal remaining on the substrate at equilibrium conditions is dependent upon the relative thickness of coating metal and substrate and their relative heat conductivities. Under the near adiabatic conditions existing in the evacuated coating chamber, heat losses are minimized and the heat supplied to the substrate by coating metal condensation is distributed throughout the coating and substrate in accordance with the known laws of heat fiow.

Re-evaporation is the factor limiting the thickness of United States Patent 0 ice vacuum vapor deposited coatings of a number of metals in addition to zinc. Each of these metals, in common with zinc, has a vapor pressure at its melting point greater than the vapor pressure in the coating chamber. We include these metals under the designation high vapor pressure metals. The more useful metals in this group in addition to zinc are cadmium, chromium, magnesium and nickel. The factor limiting vacuum vapor coating with a metal such as aluminum or tin which has at its melting point a vapor pressure less than the vapor pressure in the coating chamber is its melting point. When the coating metal reaches its melting point it fuses, loses the matte appearance characteristic of vacuum vapor deposited coatings and becomes similar to a hot dip coating. We do not consider such metals to be high vapor pressure metals. The melting points and vapor pressures of metals are conveniently tabulated in the paper Vapor Pressure Data for the More Common Elements by Richard E. Honig, published in RCA Review for June 1957, pp. -204.

We have further discovered that the re-evaporation of zinc or other high vapor pressure coating metal can be minimized by modifying the surface of the coating metal immediately after it has condensed on the substrate, and we have also found ways of conveniently effecting such modification.

It is an object of our invention, therefore, to provide a process of coating a substrate with a high vapor pressure metal by vacuum vapor deposition so as to minimize its re-evaporation. It is another object to provide such a process in which the surface of the coating metal immediately after deposition is modified to a form exhibiting relatively low vapor pressure. It is another object to bring about this modification by reacting the coating metal immediately after deposition with a surface modifying agent. Other objects of our invention will become evident in the course of the following description thereof.

In one embodiment of our invention, we carry out the vacuum vapor deposition process in the usual way to produce a coating of condensed metal which without further treatment would begin to re-evaporate. We then direct a stream of surface modifying agent upon the surface of the condensed coating metal so as to convert the surface layer of the coating into a compound of the metal having a relatively low vapor pressure. We have found that the oxide of a metal has a vapor pressure appreciably lower than the metal itself, so that an oxidizing gas is a suitable surface modifying agent. Thus, oxides of zinc, cadmium, chromium, magnesium, and nickel are representative of suitable surface modifying agents.

The practice of this embodiment of our invention may be better understood by reference to FIG. 1 of the attached drawing. Steel strip 1, the substrate here, is unwound from an uncoiling reel 2, passed through an evacuated chamber 4, and recoiled on coiling reel 3. In chamber 4 is a crucible 5 heated by an electrical resistance heater 6. Crucible 5 is positioned below the path of travel of the strip 1. Crucible 5 contains molten coating metal 7, for example, zinc, which evaporates in the evacuated chamber 4 into metal vapor 8. This vapor condenses on the colder strip 1 as it passes over crucible 5. Adjacent crucible 5 inside chamber 4 is a pipe 9 from which an oxidizing gas such as air is directed onto the surface of the condensed coating metal. At its condensation temperature, that metal reacts very rapidly with air or other oxidizing gas to form an oxide of the metal over its surface.

It will be appreciated that the gas introduced into chamber 4 through pipe 9 must be pumped out by the evacuating apparatus, not shown, in order that the necessary vacuum be maintained. Therefore, the volume of this gas is kept to a minimum and the pipe 9 is positioned so as to discharge the gas directly against the coated surface of the strip 1.

In another embodiment of our invention, we carry out the vacuum vapor deposition process in the usual way to produce a coating of condensed high vapor pressure coating metal and then condense thereon a second vaporized metal having a vapor pressure lower than the first. Where the coating metal is zinc, the second metal is preferably aluminum or tin.

The practice of this embodiment of our process may be better understood by reference to FIG. 2 of the attached drawing, in which elements like those in FIG. 1 have the same reference characters. Evacuated chamber 4 in FIG. 2 is not provided with the pipe 9 of FIG. I, but in its place has a second crucible 10 positioned adjacent crucible 5 and downstream therefrom, below the path of travel of the strip 1. Crucible is heated by electric resistance heater 11. Crucible 10 contains a second molten metal 12 which is maintained at its evaporation temperature and evaporates into vapor 13. The strip 1 after passing over crucible 5 and being coated with the condensed coating metal 7 evaporated therefrom passes immediately over crucible 10 and the second metal 12 evaporated therefrom condenses on the surface of the first coating metal 7. If second coating metal 12 is selected to have a lower vapor pressure than first coating metal 7, the re-evaporation of the vapor deposited coating within chamber 4 is correspondingly reduced.

In embodiment of the process of our invention for zinc coating which we have practiced, the coating chamber is continuously pumped to a pressure between 1 and 4X10- mm. of Hg. The substrate in the form of steel strip is passed into the coating chamber and over a crucible in which zinc is heated and vaporized. To produce thick coatings of zinc the crucible temperature is raised until the vapor pressure of the zinc immediately over the crucible reaches values between 0.2 and 3.0 mm. of Hg. This high pressure region is confined to the immediate vicinity of the crucible. The vaporized zinc condenses on the substrate and its condensation releases heat which raises the substrate temperature to values which may exceed 500 F. A zinc vapor pressure of 0.2 mm. of Hg corresponds to a zinc temperature in excess of 800 F. so that if the strip temperature stays below that value, the zinc condenses and does not re-evaporate from the strip while it is passing over the crucible. As the zinc coated strip travels away from the localized high pressure region, however, into the lower pressure of the remainder of the chamber, the zinc coating, in the absence of our process, will reevaporate. At a substrate temperature of 554 'F., for example, zinc has a vapor pressure of 10- mm. of Hg, which is above the pressure in the coating chamber. If no steps are taken to put an end to this re-evaporation, it will continue until the strip temperature falls below the value at which the vapor pressure of zinc equals the pressure in the coating chamber. In an evacuated chamber strip cools slowly because the only significant heat loss is by way of radiation, and, prior to our invention, the strip leaving the chamber had a zinc coating considerably thinner than it had when it passed over the zinc source.

By the practice of our invention which we have described, a top coating of a metal or a metal oxide which has a vapor pressure lower than that of zinc is provided on the zinc coated substrate immediately after the latter has passed over the vapor source. This top coating is selected so that it does not re-evaporate in the coating chamber at the highest temperature reached by the strip and the coating metal thereon, and it thus puts an end to coating metal re-evaporation. As a result, the coated strip leaving the coating chamber has a thicker coating of zinc under the top coat than it would have had if the top coating had not been applied.

In our invention, we prefer not to preheat the substrate before coati g. Because the substrate is a conductor of heat, the heat released by condensation of the coating metal flows to some extent into the substrate upstream of the crucible, but this preheating is unavoidable and is not the preheating with which we are concerned. We do not preheat the substrate in any way other than that incidental to condensation of the coating metal. Under some conditions We precool the substrate to a temperature below room temperature before exposing it to the vaporized coating metal. We find it advantageous under some conditions to heat the substrate subsequent to coating to a temperature sufiicient to bring about alloying between the coating and the substrate.

We claim:

1. The method of vacuum vapor coating a metal substrate with a coating metal comprising passing the metal substrate through an evacuated chamber over a source of coating metal vapor, said coating metal having a vapor pressure at its melting point greater than the vapor pressure of the evacuated chamber, adjusting the rate of evaporation of the coating metal so as to increase its vapor pressure immediately over the source above the pressure of the evacuated chamber, thereby to increase the rate of condensation of the coating metal on the substrate and the temperature of the coating metal on the substrate to a temperature above that at which the vapor pressure of the coating metal equals the pressure in the evacuated chamber, passing the coated substrate away from the source into a region of the evacuated chamber of pressure lower than the vapor pressure of the coating metal on the substrate, where the coating metal would commence to re-evaporate from the substrate if permitted to do so, and in that region providing on the surface of the coating metal a surface layer of a substance selected from the group consisting of metals and metal oxides which have vapor pressures lower than the vapor pressure of the coating metal, the surface layer being of a thickness sufficient to cover the coating metal surface so as to prevent substantial re-evaporation thereof, thereby preventing such re-evaporation and maintaining the thickness of the coating metal on the substrate leaving the evacuated chamber at a value greater than it would be in the absence of the surface layer.

2. The process of claim 1 in which the coating metal is zinc and the surface layer is zinc oxide.

3. The process of claim 1 in which the coating metal is zinc and the surface layer is aluminum.

4. The process of claim 1 in which the soating metal is zinc and the surface layer is tin.

5. The process of claim 1 in which the coating metal is zinc and the pressure in the evacuated chamber is not less than about l0 millimeters of mercury.

6. The process of claim 5 in which the vapor pressure of the zinc immediately over the source is not less than about 0.2 millimeter of mercury.

7. The process of claim 1 in which the coating metal is selected from the group consisting of zinc, cadmium, chromium, magnesium, and nickel and the surface layer is selected from the group consisting of zinc oxide, cadmium, oxide, chromium oxide, magnesium oxide, and nickel oxide.

References Cited UNITED STATES PATENTS 2,405,662 8/1'946 McManus et a1. 1l7107 X 3,322,577 5/1967 Smith, Jr. 148-63 3,255,035 6/1966 Clough et al. l17107.1 X 2,665,229 1/1954 Schuler et al. 117-107.1 X 2,812,270 11/1957 Alexander l1750 3,438,754 4/1969 Shepard et al. 117107 X ALFRED L. LEAVITT, Primary Examiner US. Cl. X.R.

UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION 3,761,302 September 25, 1973 Patent No. Dated Irwin I. Bessen et a1. Inventor(s) It is certified that error appears in the aboveidentified patent and that said Letters Patent are hereby corrected as shown below:

The ass'ignee should read 1+ Jones and Laughlin Stell Corporation,

Pittsburgh Pa.', a corporation of Pennsylvania Signed and vsealed this 17th day of September 1974.

(SEAL) Atte'st: I I

McCOY M. GIBSON JR. c. MARSHALL DANN Attesting Officer Commissioner of Patents FORM PO-1050 (10-69) I 'uscoMM-oc 60376-1 99 U,S. GOVERNMENT PRINTING GFFICE 2 1989 0-355-33L 

